1. Field of the Invention
The present invention relates to an image forming apparatus, which includes a charging member that is provided contactably to an image bearing member and charges the image bearing member, such as a laser printer, a copying machine, or a facsimile.
2. Related Background Art
Conventionally, an image forming apparatus, which uses an electrophotographic process or the like, such as a printer, a copying machine, or a facsimile is provided with a charging apparatus that charges a surface of an image bearing member.
Examples of such charging apparatuses include a charging apparatus of corona discharging system and a charging apparatus of contact charging system which charges a surface of a photosensitive member through application of a voltage to a charging member in a state in which the charging member such as a charging roller is in direct contact with the photosensitive member.
Further, the above-mentioned contact charging apparatuses have a DC application system (what is called a DC charging system) in which a voltage to be applied includes only a direct-current component and an AC+DC application system (what is called a AC charging system) in which both an alternating component and a direct-current component are applied. The DC charging system and the AC charging system each include a constant current system and a constant voltage system.
Under the comparison between the DC charging system and the AC charging system in the contact charging apparatus, the DC charging system has a disadvantage that, when a resistance value of the charging member varies due to an environmental change on a temperature, moisture, or the like, a charging potential of a surface of the photosensitive member to be charged is prone to vary accordingly. Therefore, in the case of using the DC charging system, since it has been essential that the temperature or moisture in the environment where the charging apparatus is placed is always detected, and a voltage or current to be applied to the charging member is controlled based on the detected information, there has been a problem in that the DC charging system charging apparatus becomes complicated by being equipped with a temperature sensor or moisture sensor.
As the above-mentioned DC charging system charging apparatus, there is proposed one in which, based on the measurement of a temperature and moisture, a constant current control is conducted under a certain environmental condition while a constant voltage control is conducted under another environmental condition, thereby avoiding the formation of an abnormal image (refer to, for example, Japanese Patent Application Laid-Open No. H09-179383).
Further, in comparison to the AC charging system, the DC charging system presents a problem in that image failure due to charging failure is prone to develop even in the case where slight contamination of the charging member is generated by a lubricant between a cleaning member and an image bearing member or a waste developer that has passed through the cleaning member due to cleaning failure.
On the other hand, in comparison to the DC charging system, in the AC charging system, stable charging property can be obtained without large influence due to environmental fluctuation of the temperature, moisture, and the like.
However, it is known that, when AC charging is used, an abrasion amount of a thickness of a surface layer of the image bearing member is increased compared with the case of using DC charging. In order to suppress the increase of the abrasion amount to as low a level as possible, there is proposed an image forming apparatus in which control is performed such that AC charging is used at the time of image forming while the system is switched to DC charging at the time of non-image forming (refer to, for example, Japanese Patent Application Laid-Open No. 2000-39757).
On the other hand, as a conventional example with the purpose of improving cleaning property irrespective of a charging system, there is also proposed an image forming apparatus in which control is performed such that backward rotation is prolonged (refer to, for example, Japanese Patent Application Laid-Open No. H11-219086).
The AC charging system is more advantageous for obtaining stable charging property with the above-described simple structure. However, on the contrary, in the AC charging system, the surface of the image bearing member, which serves as a member to be cleaned, vibrates due to the applied AC component, chattering of the cleaning member develops, and thus, a transfer residual developer becomes difficult to be removed by cleaning means, which may lead to a case where cleaning property of the image bearing member is not sufficiently obtained. In particular, when a spherical developer is used, there may be a case where a collecting property of the cleaning member is not sufficiently obtained.
Further, the control method, which is described in the conventional example and in which switching is performed such that AC charging is used at the time of image forming while DC charging is used at the time of non-image forming, has an effect to the above-described problem in a sense that cleaning property is improved because the system is switched to DC charging in the intermittent one-sheet image output. However, the original object of the image forming apparatus is the prevention of abrasion of the surface layer of the image bearing member. Therefore, there still remains a problem in that, in the case where a non-image region does not exist between images in continuous printing, and thus, high-printing images are continuously output or in the case where a non-image region does not exist between images in continuous printing, and thus, high-printing images are continuously output in a multi-color or color image forming apparatus that uses an intermediate transferring member, a large amount of secondary transfer residual developer, which is inversely transferred from the intermediate transferring member to the image bearing member, cannot be fully cleaned by the cleaning member, which leads to the occurrence of image failure.